Both cards aren't really for heavy gaming, but they are fine for an occasional game.
The 9600 GT performs better than HD 4670 but only when it is DDR3.
I haven't seen benchmarks for 9600 GT DDR2 but it may be at about the same level of HD 4670 DDR3.
Try to stay away from all DDR2 cards.
Also as a note, these cards can't really benefit from 1 GB of ram, so 512 MB DDR3 will be much better than 1 GB DDR2.
For the compatibility, both cards are compatible with your system.
For the CPU, why did you choose the Q8300? Is there any specific reason? There are better options such as AMD Phenom II which offers higher performance for about the same price, or Athlon II which is significantly cheaper but not really slower than Q8300, or you can step up to core i5 750 which is much better and faster but costs a bit more.
You should be fine with a Radeon 4670 with GDDR3. The 4670 is rather miserly with power and should do fine on your system with your current PSU. If you want to use the 9600GT make sure that your PSU is of decent quality, and if it isn't or you're not sure, then I would recommend you replace it.
You may also want to consider the GT240 as it's a little faster than the 4670 and uses less power. It's more expensive though ^_^